Many industrial, food and biotechnology companies use micro- and ultrafiltration equipment and methods in the processing of solutions. As examples, filtration is used as a sterilising step to remove bacteria, as a clarification step to remove suspended solids and contaminants, as a concentration step for proteins and other macromolecules or as a purification step to eliminate unwanted micro-molecules such as salts. Alternative membranes and porosities are used to suit specific applications and process requirements.
Especially for larger volumes the so called crossflow or tangential flow technique is used.
Filter elements for this type of filtration in the form of membranes (e.g. of the spiral wound, hollow fibre or flat type) are mounted in pressure resistant housing to form filtration modules or cells. In a filtration system including such cells a pump is used to feed the solution to be filtered through the cell tangentially across the membrane surface. The speed of solute filtration is governed by a number of parameters like general membrane characteristics and porosity, pressure and the level of fouling that occurs on the membrane surface.
Problems related to gel polarisation or fouling of the membrane which greatly reduces the speed of filtration have been major handicaps in the development of ultrafiltration techniques. These problems are caused by several factors, the most important of which are the formation of a gel layer and the accumulation of retained particles on the membrane surface which results in a partial blockage of the membrane pores during solute filtration. The phenomenon frequently results in a tenfold or greater reduction in membrane hydraulic permeability when compared to the original pure water permeation rate. The ultimate impact of these problems is the need for significantly increased operating pressure and membrane area requirements for a given filtration capacity, increasing hold up losses in the system i.e. losses of concentrated sample which can not be drained from the cell, adding cost and finally making filtration less competitive than alternative processing techniques.
Important parameters that also need to be taken into account in the design of filtration cells include minimising liquid hold up volume per membrane unit area, a low pressure drop across the length of the flow path, ease of cleaning with minimum dead spaces, ability to fully drain the cell, ease of scaling up or down to large capacity or small pilot systems, minimum energy requirement which means high flow rate in combination with low pressure drop across the cell and overall economy.
The use of high flow, long, thin channel configurations using either membranes in the form of a flat plate or hollow fibre bundles have shown improvements in reducing fouling whilst achieving low hold up and energy requirements.
The present invention is directed to a cells and systems making use of flat membranes.
In a known filtration cell a flat, thin channel is arranged in a spiral configuration on one side of a circular membrane. Filtrate outlet and inlet ports are fitted at the centre and the outer edge of the cell respectively. The outlet port at the centre is arranged perpendicular to the flat channel and the membrane.
The major problems associated with this configuration, is the poor utilisation of available membrane area when cutting the membrane (circular) which of course increases the cost of the membrane. Additionally this type of cell is not suitable when scaling up to large size process systems due to the central outlet which does not allow the stacking of multiple cells. For these reasons this type of configuration has been mostly limited to small laboratory systems.
Another known filtration cell using membranes of the flat type is sold by the company MILLIPORE under the trademark MINITAN. This cell has an essentially rectangular membrane which is swept by sample solution flowing through multiple straight parallel channels from an inlet manifold at one edge of the filter membrane to an outlet manifold at an opposite edge of the filter membrane.
Another problem with existing tangential flow cells relates to the need to control back pressure at the cell recirculation outlet in conjunction with pump speed setting in order to create suitable transmembrane pressure within the cell itself.
In existing systems, a valve is used to restrict flow whilst pump speed is also varied until adequate pressure is achieved. In small laboratory systems a so called pinch valve is typically used to compress the outlet tube whilst larger process systems use more sophisticated mechanical valves. The pinch valve is inexpensive but is difficult to accurately control whilst mechanical valves are more precise but add a significant cost to the overall process. In both cases, the interdependency between valve and pump setting makes process control complicated and time consuming.
To achieve adequate pressure control, at least one pressure gauge must also be fitted to the filtration system, usually at the inlet or outlet of the filtration cell. Diaphragm valves which are frequently used for this purpose result in a large liquid dead volume entering the internal mechanism which is difficult to drain with negative consequences particularly when sanitary operation is essential. Sanitary designs that have been developed to reduce this problem are expensive and cannot normally be justified in a small laboratory system. In addition, due to the pulsation effect produced by most pumps, pressure readings fluctuate at a high frequency making visual control difficult and inaccurate.
In addition to minimising hold up volume in the filtration cell itself, it is just as important many times to reduce volume and surface area in the recirculation loop including the feed reservoir, pump and connective tubing, so as to allow a high level of final concentration and/or maximum filtrate volume. As initial volumes are frequently large, the feed or sample reservoir must usually be placed at a significant distance from the cell and pump assembly. This requires additional tubing for recirculation which adds to the hold up volume and it is difficult to pump liquid completely from a large reservoir so that not all the initial sample can always be processed.
When e.g. processing small volumes of sample liquid and the retentate has a high value it is clearly of interest to keep the hold up volume of the cell as well as the complete recirculation loop at a strict minimum. Many times you have this situation when processing samples in a laboratory. On the other hand when the processed product is the filtrate and the value lost in the hold up volume is of less importance, e.g. when clarifying fruit juices, the actual hold up volume is not critical.